Phase transitions in C2O4H NH4 1/2 H2O : a light scattering study of the high pressure phase J.L. Godet, M. Krauzman, R.M. Pick, H. Poulet et N. Toupry J. Phys. France, 50 13 (1989) 1711-1728 DOI: 10.1051/jphys:0198900500130171100